Finding Reversed CT?

[big john]

I have a CT reversed on a 3-phase power meter. Unfortunately I can't shut down or actually get to the CTs themselves. Is there any way electrically to determine which CT is backwards just by testing the secondaries?

The PT secondaries are also present if that helps. Thanks.

-John

 

[gar]

110902-0952 EDT

The answer is probably yes. Depends upon the test equipment you have available, and the wiring.

An assumption is that the power flow on all phases is from a generating source to a consuming load. This means that at the load there is no generating means on one phase.

You can use a clamp-on current sensor if the current carrying wire is available. Then do a phase comparison between said sensor and the CT output. Means you need to define an orientation for said sensor based on some reference CT that you test on the bench. Easiest to do all this with a scope.

Without the extra current sensor, but using a scope, compare the CT output phase relationship to the voltage of that phase. Do this on each phase and see where the disparity is.

If you need a more detailed description I can provide that if you can define the limitations. Limitations might be the line in question is too high a voltage to work with directly, can not put a current sensor around the line the CT is located on, etc.

.

 

[ATSman]

Phase Angle Meter and more

Phase Angle Meter and more

You can do as Gar suggested or rent a Phase Angle Meter. It is an instrument that measures the phase angle between the voltage and current in each of the three phases of the system. It has connections for voltage and current and designed to be used when the system is energized and carrying load. You should have the V and A terminals at the back of the power meter. The typical system shows the voltage lagging the current (inductive, PF = 0.8) The phase that shows the voltage leading the current is the reversed CT and you can correct by reversing the secondary wires for that phase at the power meter.
Another method to check CT polarity is called the "Flick" test but the system has to be shut down to perform it. You connect a DC analog ammeter across the CT secondary and pass a wire through the CT window (it works on inline CT as well but you have to have access to the phase conductors.) Momentary touch the wire across the terminals of a battery (any voltage will work but I use a 9V) and observe the polarity. If you touch the positive side of the wire entering the H1 side of the CT the needle will "flick" positive to the right with the X1 connected to the positive terminal of the meter. If the needle deflects negative then the CT is installed backwards and can be corrected by reversing the secondary wires at the power meter. Remember the CT Polarity Definition: when instantaneous current enters the H1 primary side, instantaneous current leaves the X1 secondary terminal. It is a simple but effective test we use during startup and commissioning before the gear is energized. If there are numerous CT's to be tested then we use a CT Polarity Test Set which uses the same principal but is much quicker.
Good luck.
(someone check my leading and lagging, I think it's correct )

 

[hillbilly1]

Try this, if this is connected to a digital wattmeter, on the shorting block, install the shorting jumpers on all but one CT, move to the next one and do the same, The one that is reading a negative KW is the one that is backwards.

 

[ATSman]

Try this, if this is connected to a digital wattmeter, on the shorting block, install the shorting jumpers on all but one CT, move to the next one and do the same, The one that is reading a negative KW is the one that is backwards.

Sounds like a very clever diagnostic tool, have you done it before? It is like your are treating a 3-phase device like single phase. By shorting out two phases you are applying current to only one phase of the meter. How can it indicate a proper KW reading if it is expecting 3 currents 120 degrees apart? If it does you are assuming all meters are designed the same.
Regardless, I am curious to see what the outcome will be.